1. Field of the Invention
The invention relates to a metal fine particle for conductive metal paste that is used for the formation of an electronic circuit formation, solder materials, plating materials and the formation of a wire shielding layer etc. Also, the invention relates to a conductive metal paste and a metal film using the metal fine particle.
2. Description of the Related Art
A metal fine particle means a metal particle having a particle size of about 1-100 nm, and a phenomenon that a melting point is depressed due to a rapid increase in a surface area with respect to particle volume (hereinafter, referred to as melting point depression) is known in metal fine particle. Therefore, diffusion of the metal fine particles at an interface between particles occurs at a temperature lower than the melting point of bulk metal, and a metal bond is formed by progression of fusion (See, e.g., non-patent literary document of Ink-jet Wiring of Fine Pitch Circuits with Metallic Nano Particle Pastes, published by CMC Publishing CO., LTD. in 2006).
A simple metal fine particle is very unstable, and aggregation or fusion of particles proceeds even at around room temperature. Therefore, it is essential to suppress the aggregation or fusion of metal fine particles by coating the surface thereof with an organic substance called protective agent which exhibits adsorption properties. A conductive metal paste, which is a paste composition composed of a metal fine particle for conductive metal paste of which surface is coated with a protective agent and a solvent composition, can be sintered at low temperature using the melting point depression of the metal fine particle, and a conductive metal film can be formed therefrom. However, there is a problem that a metal film after sintering of a conductive metal paste has generally low adhesion to a base material. The adhesion varies as follows depending on the type of base material.
When the base material is metal, there is a case that a metal bond or an alloy layer is formed at an interface between the sintered metal and the metal in the base material, which improves problems of adhesion in some degree. However, when the metal bond or the alloy layer is not formed, the adhesion is inferior to that of a metal film which is formed by vapor-deposition technique or a plating method, and film adhesion required for practical use is often unsatisfied.
Meanwhile, when the base material is ceramic, the adhesion may be ensured by formation of a solid solution layer of metal and ceramics, which is called cermet layer, at an interface between the metal film and the ceramics. However, the adhesion is low in a combination of metal and ceramics without formation of the cermet layer.
When the base material is a macromolecule, different types of substances are merely physically in contact at the interface between the sintered metal and the base material, and the adhesion is hardly obtained.
Following prior arts exist in order to solve the problems of adhesion. Conventional methods are classified broadly into a method for improving a conductive metal paste itself and a method for improving a base material.
The method for improving a conductive metal paste includes a means in which a small amount of compound having adhesion to the base material such as another metal fine particle, ceramic particle or binder resin is added to the conductive metal paste (hereinafter, referred to as prior art 1).
The method for improving a base material includes a method in which a base having adhesion is preliminarily formed on a base material, on which a conductive metal paste will be applied, by using another metal paste, ceramic paste or organic paste, etc., (hereinafter, referred to as prior art 2). In addition, there is a method of changing the nature of the surface of the base material, which includes a method for modifying a surface by chemical treatment and a method in which an anchor effect is obtained by physically roughening the surface of the base material using an atmospheric-pressure plasma method (hereinafter, referred to as prior art 3).